Detailed description of the presently preferred embodiments

ABSTRACT

The present invention is a nitride compound semiconductor laser, in which a cleaved end face is flat, and a breakdown of a laser end face induced during an operation can be suppressed, which consequently enables a life to be prolonged. In the nitride compound semiconductor laser, a stress concentration suppression layer is formed between an active layer and a cap layer.

TECHNICAL FIELD

The present invention relates to a nitride semiconductor lasercharacterized by having a stress concentration suppressing layer betweenan active layer and a cap layer.

BACKGROUND ART

A GaN-based group III-V compound semiconductor (hereafter, referred toas a GaN-based semiconductor), which is a direct transitionsemiconductor whose forbidden band gap is in a range from 1.9 eV to 6.2eV, enables a realization of a semiconductor light emitting device, suchas a semiconductor laser diode (LD), a light emitting diode (LED) andthe like, in which a light emission can be obtained from a visibleregion to a ultraviolet region. Thus, in recent years, development inthe field has been vigorously advanced. Among them, actual usage of ablue-violet semiconductor LD from which a light having a light emissionwavelength of about 400 nm is obtained is especially required in orderto improve a recording density of an optical disc or the like, in afield of an optical recording. Also, a blue semiconductor LD having alight emission wavelength of about 460 nm is expected to be applied to alaser display. Moreover, an ultraviolet semiconductor LD having a lightemission wavelength of 380 nm or less is expected to be applied to alight source for phosphor excitation.

Those GaN-based semiconductor light emitting devices are typicallyprovided with GaN-based semiconductors grown on a substrate.Conventionally, as the substrate on which this GaN-based semiconductoris grown, there is no proper substrate having an excellent latticematching property with GaN. Thus, a sapphire substrate is mainly used.However, a lattice mismatching with the GaN and a thermal expansioncoefficient difference from it are very large. In this way, when thelattice matching with the substrate is poor and the thermal expansioncoefficient difference from the substrate is large, influence on acrystalline of a GaN-based semiconductor layer grown on the substrate issevere. Hence, a large quantity of dislocations such as an order of 10⁸to 10¹⁰/cm² is implanted into the GaN-based semiconductor layer in orderto relax that distortion. Among them, a threading dislocation especiallytransmitted in a thickness direction of a film is harmful for an activelayer of a device formed near a film surface, and it acts as a currentleakage portion and a non-light-emission center. So, the threadingdislocation is known as the one which damages electrical and opticalproperties of the device.

Thus, in order to manufacture the GaN-based semiconductor device, thethreading dislocation must be reduced as much as possible. In recentyears, as a method of reducing the threading dislocation, a method hasbeen employed in which a lateral direction growth for epitaxial, whichis represented by an ELO (Epitaxial Lateral Overgrowth) method. Thepresent inventors employed the ELO method and tried to reduce adislocation density of a GaN epitaxial film. Then, the dislocationdensity within a Wing portion (a portion resulting from the lateraldirection growth) under an optimized condition could be reduced to anorder of about 10⁶/cm² or less. As a result, in the GaN-basedsemiconductor laser manufactured by the present inventors, it is evidentthat a device life is improved such as about 200 hours under 50° C. and30 mW.

However, even in the case of the above-mentioned GaN-based semiconductorlaser, it is difficult to say that the actual usage can be sufficientlyattained. There is a margin for further improvement so as to improve thedevice life.

A subject of the present invention is to provide a nitride semiconductorlaser, in which a cleaved end surface is flat, and a breakdown of alaser end surface induced during an operation can be suppressed whichresults in a long life.

DISCLOSURE OF THE INVENTION

When a nitride semiconductor laser end surface after degradation wasanalyzed by using a scanning electron microscope (SEM), a transmissionelectron microscope (TEM) and the like, the present inventors found thatthe usage thereof caused the breakdown in the vicinity of the activelayer of the laser end surface, and discovered that this was one of themain reasons for the degradation of the nitride semiconductor laser.

After carrying out the eager discussion to solve the above-mentioneddegradation reasons in view of such knowledge, the present inventorsobtained an idea that the implantation of a stress concentrationsuppressing layer between an active layer and a cap layer enables thebreakdown in the vicinity of the active layer on the laser end surfacecaused by the usage to be suppressed, which can improve the life of thenitride semiconductor laser.

More specifically, by having the lattice constant graded and thecomposition of the stress concentration suppressing layer from an activelayer side to a cap layer side within the layer so that the latticeconstant and the composition of the active layer are smoothly changed tothose within the cap layer, it is possible to suppress the stressconcentration in the boundary between the active layer and the caplayer. Consequently, there were obtained unexpected ideas that thesemiconductor laser whose cleaved end face is flat can be obtained, thatthe end face degradation breakdown induced during the operation can beavoided and that the life of the nitride semiconductor laser can beimproved.

The course when the above-mentioned idea was obtained will be describedbelow in detail. The present inventors investigated in detail thebreakdown situation of the laser end surface after the degradation. FIG.2 shows a diagrammatic view of a section parallel to stripes of a frontend face. An end face breakdown 16 was induced on a front end face 15 sothat a crystal was scooped away in the vicinity of an active layer 7.This scooping of the crystal was the largest in the boundary between theactive layer 7 and a cap layer 8.

Also, the condition of the end face before the deterioration of thesemiconductor laser was investigated in detail by using the TEM and thelike. FIG. 3 shows a diagrammatic view of the section parallel to thestripes of the front end face. The possible case of the introduction ofa step 17 of about several nm or less in the boundary between the activelayer 7 and the cap layer 8 was found out on the front end face 15.

The above-mentioned stage difference 17 is induced so that the stressconcentration caused by the lattice mismatching between the crystals ofthe active layer 7 and the cap layer 8 is relaxed. Then, the possibilitythat the step 17 and the excessive stress concentration in the boundarybetween the active layer 7 and the cap layer 8 cause the end facebreakdown 16 to be especially centrally induced in the boundary betweenthe active layer 7 and the cap layer 8.

More specifically, the active layer 7 is constituted by a multiplequantum well (MQW) structure in which a composition of a well layer isGa_(0.92)In_(0.08)N and a composition of a barrier layer isGa_(0.98)In_(0.02)N, and the cap layer comprises a mixed crystal ofAl_(0.15)Ga_(0.85)N. The lattice mismatchings between the a-axis of theGaN and the free standing mixed crystals of the Ga_(0.92)In_(0.08)N, theGa_(0.98)In_(0.02)N and the Al_(0.15)Ga_(0.85)N are +0.889%, +0.222% and−0.358%, respectively. That is, in the boundary from the active layer 7to the cap layer 8, the lattice mismatching with the GaN is sharplychanged from a plus to a minus, which results in the densestconcentration of the stress.

On the basis of the discussion result as mentioned above, the presentinventors obtained the following knowledge. That is, by suppressing theexcessive stress concentration caused by the lattice mismatching betweenthe cap layer and the active layer in the vicinity of the active layer7, it is possible to manufacture the semiconductor laser in which thestep 17 does not substantially exist in the boundary between the activelayer 7 and the cap layer 8, namely, the semiconductor laser in whichthe cleaved end face is flat. Also, the end face degradation breakdowninduced during the operation can be avoided by suppressing the excessivestress concentration in the vicinity of the active layer on theabove-mentioned laser end face and by making the cleaved end face flat.

Moreover, the present inventors eagerly discussed a specific method tosuppress the excessive stress concentration in the vicinity of theactive layer. Then, there was obtained an idea that the excessive stressconcentration in the vicinity of the active layer can be suppressed byinserting a stress concentration suppression layer constituted by anAl_(x)Ga_(1-x-y)In_(y)N (1>x>0, 1>y>0 and 1>x+y>0) mixed crystal layerbetween the active layer and the cap layer, and by making itscomposition on the active layer side equal to the composition of thebarrier layer of the active layer, namely, the Ga_(0.98)In_(0.02)N, andon the other hand, by making its composition on the cap layer side equalto the composition of the cap layer, namely, the Al_(0.15)Ga_(0.85)N,and then by having the composition of the stress concentrationsuppression layer graded so as to be smoothly changed from the activelayer side to the cap layer side.

This will be described below in detail with reference to a diagrammaticview of a band structure in the vicinity of the active layer shown inFIGS. 4A to 4C. FIG. 4A shows the diagrammatic view of the bandstructure in the vicinity of the active layer of the conventionalnitride semiconductor laser. In the conventional nitride semiconductorlaser, the band gap between the active layer 7 and the cap layer 8 islarge as mentioned above, and the stress concentration is induced in theboundary between the active layer 7 and the cap layer 8. On thecontrary, FIGS. 4B and 4C show the diagrammatic views of the bandstructure in the vicinity of the active layer of the nitridesemiconductor laser according to the present invention. In the nitridesemiconductor laser according to the present invention, provision of astress concentration suppression layer 47 formed as mentioned aboveenables the smooth connection from the active layer 7 to the cap layer8, which is evident from FIGS. 4B and 4C, and thereby enables thesuppression of the excessive stress concentration in the vicinity of theactive layer.

The present inventors further repeated the discussion and completed thepresent invention.

That is, the present invention relates to:

-   -   (1) a nitride semiconductor laser characterized by having a        stress concentration suppressing layer between an active layer        and a cap layer;    -   (2) a nitride semiconductor laser according to the        above-mentioned item (1), characterized in that the stress        concentration suppressing layer has a function of relaxing a        change in a band gap between the active layer and the cap layer;    -   (3) a nitride semiconductor laser according to the        above-mentioned item (1), characterized in that the stress        concentration suppressing layer has, on an active layer side,        the same composition as the active layer, and on a cap layer        side, the same composition as the cap layer, and the composition        of the stress concentration suppression layer is inclined to the        cap layer side from the active layer side; and    -   (4) a nitride semiconductor laser according to the        above-mentioned item (3), characterized in that the composition        on the active layer side of the stress concentration suppressing        layer has the same composition as a barrier layer of an active        layer having a multiple quantum well structure.

Also, the present invention relates to:

-   -   (5) a nitride semiconductor laser according to the        above-mentioned item (2), characterized in that it has an n-type        cladding layer on a side opposite to the stress concentration        suppressing layer of the active layer, and has a p-type cladding        layer on a side opposite to the stress concentration suppression        layer of the cap layer, and in that a band gap of the active        layer is smaller than band gaps of the above-mentioned n-type        and p-type cladding layers, and a band gap of the cap layer is        larger than the band gap of the p-type cladding layer;    -   (6) a nitride semiconductor laser according to the        above-mentioned item (5), characterized in that the n-type        cladding layer comprises an n-type AlGaN mixed crystal        containing Si as n-type impurities, and the p-type cladding        layer comprises a p-type AlGaN mixed crystal containing Mg as        p-type impurities;    -   (7) a nitride semiconductor laser according to the        above-mentioned item (5), characterized in that an n-type        optical guiding layer is further formed between the active layer        and the n-type cladding layer, and a p-type optical guiding        layer is further formed between the p-type cladding layer and        the cap layer;    -   (8) a nitride semiconductor laser according to the        above-mentioned item (7), characterized in that the n-type        optical guiding layer comprises an n-type GaN containing Si as        n-type impurities, and the p-type optical guiding layer        comprises a p-type GaN containing Mg as p-type impurities;    -   (9) a nitride semiconductor laser according to the        above-mentioned item (7), characterized in that an n-type        contact layer is further formed on a side opposite to the n-type        optical guiding layer of the n-type cladding layer, and a p-type        contact layer is further formed on a side opposite to the p-type        optical guiding layer of the p-type cladding layer; and    -   (10) a nitride semiconductor laser according to the        above-mentioned item (9), characterized in that the n-type        contact layer comprises an n-type GaN containing Si as n-type        impurities, and the p-type contact layer comprises by a p-type        GaN containing Mg as p-type impurities.

Also, the present invention relates to:

-   -   (11) a nitride semiconductor laser according to the        above-mentioned item (1), characterized in that the active layer        has a multiple quantum well structure, its barrier layer        comprises Ga_(1-y1)In_(y1)N (1>y1>0), the cap layer comprises        Al_(x1)Ga_(1-x1)N (1>x1>0) and the stress concentration        suppressing layer comprises Al_(x)Ga_(1-x-y)In_(y)N (1>x>0,        1>y>0, 1>x+y>0);    -   (12) a nitride semiconductor laser according to the        above-mentioned item (11), characterized in that an atom        composition ratio (x, y) of Al and In in the        Al_(x)Ga_(1-x-y)In_(y)N (1>x>0, 1>y>0, 1>x+y>0) constituting the        stress concentration suppressing layer is made graded from (0,        y1) (the y1 indicates an atom composition ratio of In in the        Ga_(1-y1)In_(y1)N constituting the barrier layer, and satisfies        (1>y1>0)) to (x1, 0) (the x1 indicates an atom composition ratio        of Al in the Al_(x1)Ga_(1-x1)N constituting the cap layer, and        satisfies (1>x1>0)), from the active layer side to the cap layer        side;    -   (13) a nitride semiconductor laser according to the        above-mentioned item (11), characterized in that the active        layer is sandwiched between the p-type cladding layer and the        n-type cladding layer, and the cap layer is sandwiched between        the p-type cladding layer and the active layer; and    -   (14) a nitride semiconductor laser according to the        above-mentioned item (13), characterized in that the n-type        cladding layer comprises an n-type AlGaN mixed crystal        containing Si as n-type impurities, and the p-type cladding        layer comprises a p-type AlGaN mixed crystal containing Mg as        p-type impurities.

Also, the present invention relates to:

-   -   (15) a nitride semiconductor laser according to the        above-mentioned item (13), characterized in that an n-type        optical guiding layer is further formed between the active layer        and the n-type cladding layer, and a p-type optical guiding        layer is further formed between the p-type cladding layer and        the cap layer;    -   (16) a nitride semiconductor laser according to the        above-mentioned item (15), characterized in that the n-type        optical guiding layer comprises an n-type GaN containing Si as        n-type impurities, and the p-type optical guiding layer        comprises a p-type GaN containing Mg as p-type impurities;    -   (17) a nitride semiconductor laser according to the        above-mentioned item (15), characterized in that an n-type        contact layer is further formed on a side opposite to the n-type        optical guiding layer of the n-type cladding layer, and a p-type        contact layer is further formed on a side opposite to the p-type        optical guiding layer of the p-type cladding layer; and    -   (18) a nitride semiconductor laser according to the        above-mentioned item (17), characterized in that the n-type        contact layer comprises an n-type GaN containing Si as n-type        impurities, and the p-type contact layer comprises a p-type GaN        containing Mg as p-type impurities.

Also, the present invention relates to:

-   -   (19) a manufacturing method of a nitride semiconductor laser,        characterized by including a step of growing a stress        concentration suppression layer on an active layer and a step of        growing a cap layer on the stress concentration suppressing        layer;    -   (20) a manufacturing method of a nitride semiconductor laser        according to the above-mentioned item (19), characterized in        that the stress concentration suppressing layer has a function        of relaxing a change in a band gap between the active layer and        the cap layer;    -   (21) a manufacturing method of a nitride semiconductor laser        according to the above-mentioned item (19), characterized in        that the stress concentration suppressing layer has, on an        active layer side, a composition identical to that of the active        layer, and on a cap layer side, a composition identical to that        of the cap layer, and the composition of the stress        concentration suppressing layer is made graded to the cap layer        side from the active layer side; and    -   (22) a manufacturing method of a nitride semiconductor laser        according to the above-mentioned item (21), characterized in        that the composition on the active layer side of the stress        concentration suppressing layer has a composition identical to        that of a barrier layer of an active layer having a multiple        quantum well structure.

Also, the present invention relates to:

-   -   (23) a manufacturing method of a nitride semiconductor laser        according to the above-mentioned item (19), characterized in        that it further includes a step of growing the active layer on        an n-type cladding layer and a step of growing a p-type cladding        layer on the cap layer, and a band gap of the active layer is        smaller than band gaps of the n-type and p-type cladding layers,        and a band gap of the cap layer is larger than the band gap of        the p-type cladding layer;    -   (24) a manufacturing method of a nitride semiconductor laser        according to the above-mentioned item (23), characterized in        that the n-type cladding layer comprises an n-type AlGaN mixed        crystal containing Si as n-type impurities, and the p-type        cladding layer comprises a p-type AlGaN mixed crystal containing        Mg as p-type impurities;    -   (25) a manufacturing method of a nitride semiconductor laser        according to the above-mentioned item (19), characterized in        that it includes a step of growing an n-type optical guiding        layer on an n-type cladding layer, a step of growing the active        layer on the n-type optical guiding layer, a step of growing a        p-type optical guiding layer on the cap layer, and a step of        growing a p-type cladding layer on the p-type optical guiding        layer;    -   (26) a manufacturing method of a nitride semiconductor laser        according to the above-mentioned item (25), characterized in        that the n-type optical guiding layer comprises an n-type GaN        containing Si as n-type impurities, and the p-type optical        guiding layer comprises a p-type GaN containing Mg as p-type        impurities;    -   (27) a manufacturing method of a nitride semiconductor laser        according to the above-mentioned item (25), characterized in        that it further includes a step of growing the n-type cladding        layer on an n-type contact layer and a step of growing a p-type        contact layer on the p-type cladding layer; and    -   (28) a manufacturing method of a nitride semiconductor laser        according to the above-mentioned item (27), characterized in        that the n-type contact layer comprises an n-type GaN containing        Si as n-type impurities, and the p-type contact layer comprises        a p-type GaN containing Mg as p-type impurities.

Also, the present invention relates to:

-   -   (29) a manufacturing method of a nitride semiconductor laser        according to the above-mentioned item (19), characterized in        that the active layer has a multiple quantum well structure, its        barrier layer comprises Ga_(1-y1)In_(y1)N (1>y1>0), the cap        layer comprises Al_(x1)Ga_(1-x1)N (1>x1>0) and the stress        concentration suppressing layer comprises        Al_(x)Ga_(1-x-y)In_(y)N (1>x>0, 1>y>0, 1>x+y>0);    -   (30) a manufacturing method of a nitride semiconductor laser        according to the above-mentioned item (29), characterized in        that an atom composition ratio (x, y) of Al and In in the        Al_(x)Ga_(1-x-y)In_(y)N (1>x>0, 1>y>0, 1>x+y>0) constituting the        stress concentration suppression layer is made graded from (0,        y1) (the y1 indicates an atom composition ratio of In in the        Ga_(1-y1)In_(y1)N constituting the barrier layer, and satisfies        (1>y1>0)) to (x1, 0) (the x1 indicates an atom composition ratio        of Al in the Al_(x1)Ga_(1-x1)N constituting the cap layer, and        satisfies (1>x1>0)), from the active layer side to the cap layer        side;    -   (31) a manufacturing method of a nitride semiconductor laser        according to the above-mentioned item (29), characterized in        that it further includes a step of growing the active layer on        an n-type cladding layer and a step of growing a p-type cladding        layer on the cap layer; and    -   (32) a manufacturing method of a nitride semiconductor laser        according to the above-mentioned item (31), characterized in        that the n-type cladding layer comprises an n-type AlGaN mixed        crystal containing Si as n-type impurities, and the p-type        cladding layer comprises a p-type AlGaN mixed crystal containing        Mg as p-type impurities.

Also, the present invention relates to:

-   -   (33) a manufacturing method of a nitride semiconductor laser        according to the above-mentioned item (29), characterized in        that it includes a step of growing an n-type optical guiding        layer on an n-type cladding layer, a step of growing the active        layer on the n-type optical guiding layer, a step of growing a        p-type optical guiding layer on the cap layer, and a step of        growing a p-type cladding layer on the p-type optical guiding        layer;    -   (34) a manufacturing method of a nitride semiconductor laser        according to the above-mentioned item (33), characterized in        that the n-type optical guiding layer comprises an n-type GaN        containing Si as n-type impurities, and the p-type optical        guiding layer comprises a p-type GaN containing Mg as p-type        impurities;    -   (35) a manufacturing method of a nitride semiconductor laser        according to the above-mentioned item (33), characterized in        that it includes a step of growing the n-type cladding layer on        an n-type contact layer and a step of further growing a p-type        contact layer on the p-type cladding layer; and    -   (36) a manufacturing method of a nitride semiconductor laser        according to the above-mentioned item (35), characterized in        that the n-type contact layer comprises an n-type GaN containing        Si as n-type impurities, and the p-type contact layer comprises        a p-type GaN containing Mg as p-type impurities.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional diagrammatic view of a conventional nitridesemiconductor laser.

FIG. 2 is a diagrammatic view showing a condition of an end facebreakdown in a conventional nitride semiconductor laser afterdegradation. Incidentally, the figure shows a cross-section parallel toa stripe direction on a laser end face.

FIG. 3 is a diagrammatic view showing a condition of an end face at atime of a cleavage in the conventional nitride semiconductor laser.Incidentally, the figure shows the cross-section parallel to the stripedirection on the laser end face.

FIG. 4A is a diagrammatic view of a band structure in the vicinity of anactive layer of the conventional nitride semiconductor laser. FIGS. 4Band 4C are diagrammatic views of a band structure in the vicinity of anactive layer of a nitride semiconductor laser according to the presentinvention.

FIG. 5 is a diagrammatic view of a cross-section vertical to a resonatorlength direction of the nitride semiconductor laser according to thepresent invention.

FIG. 6 is a view showing a graded range and an grading method of acomposition of a stress concentration suppressing layer in the nitridesemiconductor laser according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, the nitride semiconductor laser implies thesemiconductor laser provided with nitride semiconductors. Here, thenitride semiconductor is basically assumed to include the semiconductorsof all compositions in which composition ratios x, y and z in a chemicalformula of In_(x)Al_(y)Ga_(z)N (x, y, z<1, x+y+z=1) are changed withinthe respective ranges. For example, even InGaN (x=0.4, y=0, z=0.6) isincluded in the “Nitride Semiconductor”. Moreover, it includes amaterial in which a part of In, Al and Ga that belong to a group IIIelement is replaced with B (boron) and a material in which a part of Nthat belongs to a group V element is replaced with P (phosphorus). Atthis time, the group III element includes any one of the above-mentionedthree elements (In, Al and Ga), and the group V element always includesN (nitrogen). By the way, the above-mentioned GaN-based semiconductor isa concept included in the nitride semiconductor.

The nitride semiconductor laser in the present invention ischaracterized by having a stress concentration suppressing layer betweenan active layer and a cap layer. The inclusion of the stressconcentration suppressing layer enables the suppression of the stressconcentration caused by the lattice mismatching between the active layerand the cap layer. Actually, in the stress concentration suppressinglayer, the relaxation of the change in the band gap between the activelayer and the cap layer enables the suppression of the stressconcentration caused by the lattice mismatching between the active layerand the cap layer.

As the preferable embodiment of the nitride semiconductor laser in thepresent invention, a nitride semiconductor laser is listed in which ithas a stress concentration suppression layer between an active layer anda cap layer, and the stress concentration suppressing layer has, on theactive layer, a composition equal to the composition of the activelayer, and it has, on the cap layer side, a composition equal to thecomposition of the cap layer, and the composition of the stressconcentration suppressing layer is made graded from the active layerside to the cap layer side, namely, smoothly changed. As mentionedabove, by smoothly changing the composition of the stress concentrationsuppressing layer from the active layer side to the cap layer, thechange of the band gap to the cap layer from the active layer can berelaxed to thereby suppress the stress concentration caused by thelattice mismatching between the active layer and the cap layer.

Here, the active layer may have any of the known structures. However, ifthe active layer has the multiple quantum well structure, in theabove-mentioned embodiment, a composition of the active layer side ofthe stress concentration suppressing layer is desired to be equal to acomposition of a barrier layer of the active layer.

The nitride semiconductor laser according to the present invention, ifhaving the above-mentioned characteristic feature, may have a knownstructure used in that technical field.

For example, one embodiment of the nitride semiconductor laser in thepresent invention includes a semiconductor laser provided with a p-typecladding layer and an n-type cladding, having an active layer of a bandgap smaller than those cladding layers between the n-type and p-typecladding layers, a cap layer of a band gap larger than the p-type cladlayer between the p-type clad layer and the active layer, and a stressconcentration suppressing layer having a function of relaxing the changeof the band gap between the active layer and the cap layer between theactive layer and the cap layer. Here, the p-type cladding layer isdesired to be composed of nitride semiconductors containing p-typeimpurities, for example, Mg and the like. Also, the n-type claddinglayer is desired to be composed of nitride semiconductors containingn-type impurities, for example, Si and the like.

Another embodiment of the nitride semiconductor laser in the presentinvention includes, for example, a nitride semiconductor laser in whicha p-type optical guiding layer is further formed between the p-typecladding layer and the cap layer, and an n-type optical guiding layer isfurther formed between the active layer and the n-type cladding layer,in the above-mentioned embodiment.

At this time, preferably, a band gap of the p-type optical guiding layeris smaller than a band gap of the p-type cladding layer, and a band gapof the n-type optical guiding layer is smaller than a band gap of then-type cladding layer, and the above-mentioned band gaps of the n-typeand p-type optical guiding layers are larger than a band gap of theactive layer. Also, the p-type optical guiding layer is desired to becomposed of the nitride semiconductors containing the p-type impurities,for example, Mg and the like, and the n-type optical guiding layer isdesired to be composed of the nitride semiconductors containing then-type impurities, for example, Si and the like.

In the nitride semiconductor laser of the above-mentioned embodiment, ap-type contact layer may be further formed on a side opposite to thep-type optical guiding layer of the p-type cladding layer, and an n-typecontact layer may be further formed on a side opposite to the n-typeoptical guiding layer of the n-type cladding layer. Here, the p-typecontact layer is desired to be composed of the nitride semiconductorscontaining the p-type impurities, for example, Mg and the like. Also,the n-type contact layer is desired to be composed of the nitridesemiconductors containing the n-type impurities, for example, Si and thelike.

A preferable embodiment of the nitride semiconductor laser according tothe present invention includes a nitride semiconductor laser in which ithas: an active layer of a multiple quantum well structure having abarrier layer constituted by Ga_(1-y1)In_(y1)N (1>y1>0); a cap layerconstituted by Al_(x1)Ga_(1-x1)N (1>x1>0); and a stress concentrationsuppressing layer constituted by Al_(x)Ga_(1-x-y)In_(y)N (1>x>0, 1>y>0,1>x+y>0) between the active layer and the cap layer.

In the nitride semiconductor laser of the above-mentioned embodiment, anatom composition ratio (x, y) between Al and In of theAl_(x)Ga_(1-x-y)In_(y)N constituting the stress concentrationsuppressing layer is desired to be graded from (0, y1) to (x1, 0), fromthe active layer side to the cap layer side, namely, smoothly changed.By the way, the y1 implies the atom composition ratio of In in theGa_(1-y1)In_(y1)N constituting the barrier layer, and the x1 implies theatom composition ratio of Al in the Al_(x1)Ga_(1-x1)N constituting thecap layer.

This is described in specific with reference to FIG. 6. Preferably, (x,y) is (0, y1) on a boundary between the stress concentration suppressinglayer and the active layer, and it is (x1, 0) on a boundary between thestress concentration suppressing layer and the cap layer, and it furtherleads to the (x1, 0) from the (0, y1) via a blackly painted region ofFIG. 6 within the stress concentration suppressing layer. At this time,the composition of the stress concentration suppression layer is desiredto be smoothly changed within the layer. In specific, a preferableexample includes a case where the inclination of the locus of the (x, y)to the (x1, 0) from the (0, y1) is desired to be always minus, and amore preferable example includes a case where the (x, y) draws inaccordance with the locus from (1) to (3) of FIG. 6.

In the above-mentioned nitride semiconductor laser of the preferableembodiment according to the present invention, if it has theabove-mentioned characteristic features, it may have a known structureused in the technical field.

A more specific embodiment of the nitride semiconductor laser of theabove-mentioned preferable embodiment includes, for example, asemiconductor laser in which a lamination structure composed of theactive layer, the stress concentration suppressing layer and the caplayer as mentioned above is sandwiched between the n-type cladding layerand the p-type cladding layer. That is, there may be included asemiconductor laser in which the active layer is formed on the n-typecladding layer, the stress concentration suppressing layer is formed onthe active layer, the cap layer is formed on the stress concentrationsuppressing layer, and the p-type cladding layer is formed on the caplayer. Here, the compositions of the p-type cladding layer and then-type cladding layer are not especially limited. However, the n-typecladding layer is desired to be constituted by n-type AlGaN mixedcrystals to which Si as the n-type impurity is added, and the p-typecladding layer is desired to comprise p-type AlGaN compound crystals towhich Mg as the p-type impurity is added.

In the above-mentioned actual embodiment, a p-type optical guiding layermay be further formed between the p-type cladding layer and the caplayer, and an n-type optical guiding layer may be further formed betweenthe active layer and the n-type cladding layer. Moreover, a p-typecontact layer may be further formed on a side opposite to the p-typeoptical guiding layer of the p-type cladding layer, and an n-typecontact layer may be further formed on a side opposite to the n-typeoptical guiding layer of the n-type cladding layer.

Here, the compositions of the above-mentioned respective layers are notespecially limited. However, the n-type optical guiding layer is desiredto be composed of the n-type GaN to which Si as the n-type impurity isadded, and the p-type optical guiding layer is desired to be composed ofthe p-type GaN to which Mg as the p-type impurity is added. Also, then-type contact layer is desired to be composed of the n-type GaN towhich Si as the n-type impurity is added, and the p-type contact layeris desired to be composed of the p-type GaN to which Mg as the p-typeimpurity is added.

A manufacturing method of a nitride semiconductor laser according to thepresent invention may be based on a known method. In specific, thenitride semiconductor laser according to the present invention can bemanufactured by sequentially combining the processes for growing thenitride semiconductor layers constituting the nitride semiconductorlaser, under a condition that a lateral direction growth is generated.

The method of growing the nitride semiconductor layers constituting thenitride semiconductor laser according to the present invention is notespecially limited. For example, it may use a known method, such as ametal organic chemical vapor deposition (MOCVD) method, a halide vaporphase epitaxy or a molecular beam epitaxy (MBE) method or the like.

The further specific embodiment of the nitride semiconductor laseraccording to the present invention will be described below in detailwith reference to the attached drawings.

FIG. 5 is a cross-sectional view showing the configuration of thenitride semiconductor laser according to the actual embodiment of thepresent invention. Although a substrate 1 includes a sappier substrate,SiC, Si, GaAs, spinel or ZnO or the like, it is desirable to use thesappier substrate in which a c plane is centrally used. A first GaNlayer 3 having a thickness of about 1 to 5 μm, preferably, about 1 to 3μm is laminated on the substrate 1 through a buffer layer 2 composed ofa nitride semiconductor (for example, GaN, AlN or InGaN or the like)having a lamination direction thickness (hereafter, merely referred toas a thickness) of about 30 nm. As the buffer layer 2, an undoped GaNlayer is especially desired. Also, the first GaN layer 3 may be anundoped GaN layer or a GaN layer on which impurities are doped, forexample, an n-type GaN layer on which n-type impurities such as Si andthe like are doped. However, the undoped GaN layer is especiallydesired.

The substrate 1, the buffer layer 2 and a part of the first GaN layerare removed, for example, in a shape of stripes, and a second GaN layer4 is laminated thereon by the ELO method, as shown in FIG. 5. The secondGaN layer 4 is composed of n-type GaN to which Si as an n-type impurityis added, and it has a role as an n-type cladding layer.

An n-type cladding layer 5, an n-type optical guiding layer 6, an activelayer 7, a stress concentration suppressing layer 47, a cap layer 8, ap-type optical guiding layer 9, a p-type cladding layer 10 and a p-typecontact layer 11, which act as the nitride semiconductor layers, aresequentially laminated on this second GaN layer 4.

The n-type cladding layer 5 has a thickness of about 1 μm, and itcomprises an n-type AlGaN mixed crystal to which Si as an n-typeimpurity is added. The n-type optical guiding layer 6 has a thickness ofabout 0.1 μm, and it comprises an n-type GaN to which Si as an n-typeimpurity is added. The active layer 7 comprises the GaInN mixed crystalhaving the multiple quantum well (MQW) structure in which a thickness ofa well is about 3 nm and a thickness of a barrier layer is about 4 nm.The stress concentration suppressing layer 47 comprises an AlGaInN mixedcrystal in which the composition is gradually graded. The cap layer 8 isprovided in order to protect the active layer 7 from being deterioratedwhen the upper structure containing the p-type optical guiding layer isformed on the active layer 7. It comprises the AlGaN mixed crystalhaving a thickness of about 20 nm.

The p-type optical guiding layer 9 has a thickness of about 0.1 μm, andit comprises a p-type GaN to which Mg as a p-type impurity is added. Thep-type cladding layer 10 has a thickness of about 0.5 μm, and itcomprises a p-type AlGaN mixed crystal to which Mg as a p-type impurityis added. Also, the p-type cladding layer 10 may comprise a superlattice structure composed of an AlGaN layer and a GaN layer. The p-typecontact layer 11 has a thickness of about 0.1 μm, and it comprises ap-type GaN to which Mg as a p-type impurity is added. The upper portionof the p-type cladding layer 10 and the p-type contact layer 11 may beprocessed as the upper mesa structure whose cross-sectional shape istapered and striped in order to attain a current confinement.

Together with an insulating layer 12 made of insulating material such assilicon oxide (SiO₂) and the like, a p-side electrode 13 is formed onthe p-type contact layer 11 through an opening formed on the insulatinglayer 12. The p-side electrode 13 is configured such that palladium(Pd), platinum (Pt) and gold (Au) are sequentially laminated from theside of the p-type contact layer 11. By the way, this p-side electrode13 is formed in a shape of a slender band (a shape of a band extended ina direction vertical to the drawing in FIG. 5) in order to attain thecurrent confinement. Also, an n-side electrode 14 in which titanium(Ti), aluminum (Al) and gold (Au) are sequentially laminated is formedon the second GaN layer 4.

In this nitride semiconductor laser, although they are not shown,reflection mirror layers are formed on a pair of sides vertical to alength direction (namely, a resonator length direction) of the p-sideelectrode 13, respectively.

The manufacturing method of the above-mentioned nitride semiconductorlaser will be described below. The above-mentioned manufacturing methodis one of the specific embodiments of the manufacturing method of thenitride semiconductor laser according to the present invention.

This manufacturing method carries out a known pre-treatment, such as anoperation for washing a surface of the substrate 1 through a thermalcleaning and the like, depending on a desire. The buffer layer 2 isgrown on the substrate 1 by the MOCVD method. A growth temperature ofthe buffer layer 2 is desired to be a temperature lower than a growthtemperature of the first GaN layer 3 as described later, specifically, atemperature of about 520° C.

After that, the first GaN layer 3 is grown on the buffer layer 2 by theMOCVD method. The growth temperature of the first GaN layer 3 is, forexample, about 900° C. to 1100° C., preferably, about 1000° C. Also, afilm thickness of the first GaN layer 3 is not especially limited.However, it is properly set such that a concave convex structure shownin FIG. 5 can be formed. Since a cycle of the concave convex structureis desired to be about 3 to 25 μm, it is desirable to form the first GaNlayer 3 at a film thickness of about 1 to 5 μm.

After that, the substrate is taken away from an MOCVD apparatus. A maskformation film for a protective film mask formation is formed on thefirst GaN layer 3, and it is patterned to thereby form a protective filmmask (not shown) of a predetermined pattern.

In order to form the protective film mask having the predeterminedpattern, at first, for example, the technique such as a CVD method, adeposition method, a sputtering method or the like is used to form themask formation film on the first GaN layer 3 and then form a resist filmon the mask formation film. In succession, the predetermined pattern isexposed and developed to thereby form a resist pattern to which thepattern is transferred. By etching the mask formation film using theformed resist pattern, it is possible to form the protective film maskhaving the predetermined pattern.

When the protective film mask of the predetermined pattern is formed inthe above-mentioned process, the pattern is not especially limited if itis shaped such that a part of the first GaN layer corresponding to theconcave portion of the formed concave convex structure is exposed. Forexample, the shapes of stripes, zigzags, dots, grids and the like areincluded. In a case of the stripe-shaped pattern, for example, a stripewidth is desired to be about 0.5 to 20 μm, and a stripe interval isdesired to be about 1 to 25 μm. Also, a thickness of the protective filmis not especially limited. However, it is desirable to be about 1 μm orless in view of the easiness of the process.

Also, the material of the mask formation film is not especially limitedif it is the material having the property that disables the nitridesemiconductor layer to be grown on the protective film or causes thegrowth to be difficult. For example, SiO_(x), SiN_(x), TiN, TiO, W andthe like may be used.

In succession, the upper layer portion of the substrate and the firstGaN layer exposed from the protective film mask are selectively etchedand removed. After that, the protective film mask is removed. Then, theconcave convex structure having the concave portion in which thesubstrate is exposed and the convex portion composed of the first GaNlayer 3 and the upper portion of the substrate 1 is formed on thesubstrate surface.

At this step, when the upper layer portion of the substrate 1 and thefirst GaN layer 3 in the region in which the protective film mask is notformed, namely, the region exposed from the protective film mask areetched and removed, the etched amount of the substrate is desired to beabout 2 μm or less, preferably, about 0.2 μm.

The cross-sectional shape of the convex portion formed by the etchingmay be tapered. However, it is desirable to be a vertical plane.

The etching method includes the methods such as a wet etching method, adry etching method and the like. However, the dry etching method isdesired. The dry etching method includes, in specific, a reactive iondry etching (RIE) method, a reactive ion beam dry etching (RIBE) method,for example.

Again, the substrate is fed into the MOCVD apparatus and, under acondition that the lateral direction growth is generated, the n-typesecond GaN layer 4, the n-type cladding layer 5 composed of the n-typeAlGaN, the n-type optical guiding layer 6 composed of the n-type GaN,the active layer 7, the stress concentration suppressing layer 47, thecap layer 8, the p-type optical guiding layer 9 composed of the p-typeGaN, the p-type cladding layer 10 composed of the p-type AlGaN and thep-type contact layer 11 composed of the p-type GaN are sequentiallylaminated.

Here, the active layer 7 having the multiple quantum well structure inwhich a GaInN layer acts as a light emission layer is formed. Then, thestress concentration suppression layer 47, in which it is composed of anAlGaIn layer and its composition is graded, is formed. And, the p-typeAlGaN cap layer 8 is formed thereon at a relatively low temperature. TheIn composition in the active layer constituted by the GaInN multiplequantum well structure is desired to be adjusted to, for example, about0.08 in the well layer, and for example, about 0.02 in the barrierlayer. Also, it is desirable to adjust the Al composition ratio in thecap layer composed of AlGaN to, for example, about 0.15. When the activelayer and the cap layer are set in this way, in such a way that theatomic composition ratio (x, y) of Al and In in Al_(x)Ga_(1-x-y)In_(y)Nof the stress concentration suppressing layer 47 has the value of (0,0.02) equal to the composition of the barrier layer of the active layer,on the active layer side, and it has the value of (0.15, 0) equal to thecomposition of the cap layer, on the cap layer side, the stressconcentration suppression layer 47 is grown.

As growing materials of those nitride semiconductor layers, it isdesirable to use, for example, tri-methyl gallium ((CH₃)₃Ga:TMG) as araw material of Ga of the group III element, tri-methyl aluminum((CH₃)₃Al:TMAl) as a raw material of Al of the group III element,tri-methyl indium ((CH₃)₃In:TMIn) as a raw material of In of the groupIII element, and ammonium (NH₃) as a raw material of N of the group Velement.

Also, as a carrier gas, it is desirable to use, for example, a mixturegas of hydrogen (H₂) and nitrogen (N₂).

As a dopant, it is desirable to use as an n-type dopant, for example,mono-silane, and as a p-type dopant, for example,bis=methyl-cyclo-pentadienyl-magnesium ((CH₃C₅H₄)₂Mg; MeCp₂Mg) orbis=cyclo-pentadienyl-magnesium ((C₅H₅)₂Mg; Cp₂Mg).

Next, the substrate on which the nitride semiconductor layer is grown isagain taken away from the MOCVD apparatus. The insulating layer 12 madeof SiO₂ is formed on the p-type contact layer 11 composed of the p-typeGaN, for example, by the CVD method. Next, a resist film (not shown) iscoated on the insulating layer 12, and a mask pattern corresponding to aformation position of a p-side electrode 13 is formed by aphoto-lithography. After that, this is used to etch, and the insulatinglayer 12 is selectively removed, and an opening corresponding to theformation position of the p-side electrode 13 is formed.

In succession, for example, palladium (Pd), platinum (Pt) and gold (Au)are sequentially deposited on the entire surface (namely, on the p-typecontact layer 11 made of the p-type GaN from which the insulating layer12 is selectively removed and on the resist film (not shown)). Then, theresist film (not shown) together with the palladium, the platinum andthe gold, which are deposited on this resist film, is removed (liftedoff) to thereby form the p-side electrode 13.

After the formation of the p-side electrode 13, correspondingly to theformation position of the n-side electrode 14, the insulating layer 12,the p-type contact layer 11, the p-type cladding layer 10, the p-typeoptical guiding layer 9, the cap layer 8, the stress concentrationsuppressing layer 47, the active layer 7, the n-type optical guidinglayer 6 and the n-type cladding layer 5 are removed sequentially andselectively. After that, titanium, aluminum and gold are selectivelydeposited on the second GaN layer 4, and the n-side electrode 14 isformed.

After the formation of the n-side electrode 14, the substrate 1 iscleaved at a predetermined width, vertically to the length direction(the resonator length direction) of the p-side electrode 13, and thereflection mirror layer is formed on the cleaved surface. Consequently,the nitride semiconductor laser according to the present invention isformed as shown in FIG. 5.

By the way, the above-mentioned manufacturing method has been describedby limiting the growing method to the MOCVD method. However, it may begrown by using the other vapor phase growing methods, such as the halidevapor phase epitaxy or the molecular beam epitaxy (MBE) method or thelike.

In the semiconductor laser according to the present invention, theintroduction of the stress concentration suppressing layer-composed ofthe graded composition AlGaInN mixed crystal enables the suppression ofthe concentration of the stress in the boundary between the active layerand the cap layer (hereafter, referred to as the active layer/cap layerboundary). Thus, in the cleaving process required when the semiconductorlaser is manufactured, the occurrence of the stage difference in theactive layer/cap layer boundary can be suppressed on the cleaved endface. Also, in the case of the situation that the stress is concentratedin the active layer/cap layer boundary, in association with the thermalstress generation in the vicinity of the end face caused by theoperation, the end face breakdown is progressed to thereby shorten thelife of the semiconductor laser. However, in the semiconductor laseraccording to the present invention, the stress concentration in theactive layer/cap layer boundary is suppressed to thereby enable thesuppression of the progress of the end face breakdown. As a result, itis possible to prolong the life of the semiconductor laser.

1-24. (canceled)
 25. A nitride semiconductor laser including a stressconcentration suppressing layer between an active layer and a cap layer,the stress concentration suppressing layer relaxing a change in a bandgap between the active layer and the cap layer, said laser furthercomprising: an n-type cladding layer on a side opposite to the stressconcentration suppressing layer of the active layer; and a p-typecladding layer on a side opposite to the stress concentrationsuppression layer of the cap layer, wherein, a band gap of the activelayer is smaller than band gaps of said n-type and p-type claddinglayers, a band gap of the cap layer is larger than the band gap of thep-type cladding layer, an n-type optical guiding layer is further formedbetween the active layer and the n-type cladding layer, and comprises ann-type GaN containing Si as n-type impurities, a p-type optical guidinglayer is further formed between the p-type cladding layer and the caplayer, and comprises a p-type GaN containing Mg as p-type impurities,and an Al composition ratio in the cap layer is composed of AlGaN to0.15.